In general the metal plating of plastics and other nonconductive substrates is well known. The preparation of printed circuit boards requires the plating of conductive metal layers, usually copper, onto the plastic/metal composite structure of the board. Printed circuit boards vary in design and may only have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of interleaved parallel planar copper and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facilitate connection between the layers and/or the exterior surfaces.
For the most part these through-hole connections are made by utilizing an electroless metal plating cycle which consists of a sequence of steps. In this sequence the through-holes are first treated to clean and condition the surfaces of the board. This cleaning and conditioning sequence can be as simple as one process step followed by a rinse or as complicated as a full etch back or desmear cycle involving a long series of process steps and appropriate rinsing. For a further description see U.S. Pat. No. 4,756,930, the teachings of which are incorporated herein by reference in their entirety.
Following cleaning and conditioning, the surfaces are then normally subjected to activation. For electroless metallization processes, activation normally consists of contacting the boards with a pre-activator followed by a palladium-tin colloidal activator solution. For a further description see U.S. Pat. No. 4,863,758, the teachings of which are incorporated herein by reference in their entirety. The activated surfaces are then optionally subjected to an accelerator. For a discussion of accelerators, their composition and uses see U.S. Pat. No. 4,608,275, the teachings of which are incorporated herein by reference in their entirety.
Finally, the surfaces are plated in the electroless plating solution in order to deposit conductive metal, usually copper, onto the non-conductive surfaces of the board in order to make the electrical connections necessary. For a discussion of the electroless plating cycle as a whole and the electroless plating solution itself, see U.S. Pat. No. 4,976,990, the teachings of which are incorporated herein by reference in their entirety.
The preceding procedure, however, can be time consuming, costly, relatively inefficient and troubling from environmental and safety perspectives. These shortcomings have prompted industry to develop various processes to replace the conventional electroless copper process. Among these replacement processes are a variety of processes generically known as direct plate processes. Direct plate processes have at least one common attribute in that direct plate processes plate metal onto non-conductive surfaces without the use of typical electroless copper baths.
U.S. Pat. No. 3,099,608 (Radovsky, et. al.), the teachings of which are incorporated herein in their entirety, discusses an attempt to avoid the need for an electroless plating step by directly electroplating metal onto a thin layer of colloidal palladium. This palladium film was deposited onto the substrate surface utilizing a palladium-tin catalyst solution such as is described in U.S. Pat. No. 4,863,758, the teachings of which are incorporated herein by reference in their entirety.
Many improvements have subsequently been made to the basic process described in Radovsky, et al. al. One such improvement is described in Morrissey, et.al. (G.B. Pat. No. 123,0 - 36A) wherein particular electroplating bath additives are used to enhance the electroplating on the palladium (activator) film. In addition see Okabayashi (U.S. Pat. No. 5,342,501 ), the teachings of which are incorporated herein by reference in their entirety, wherein basic accelerating solutions are applied after palladium activation to increase the activity of the palladium film towards electroplating and thereby to improve the film's electroplatability.
The various direct plate processes which involve direct electroplating over a palladium film, such as the processes described in Radovsky et al., Morrisey et al. and Okabayashi, have some drawbacks or problems. One major problem is the formation of a smut on the activated surface during the initial stages of electroplating. When these palladium based direct electroplating processes are utilized, a smut forms on the activated surfaces (ie. the surfaces to be plated which have been activated with palladium or palladium-tin colloidal solutions) during the initial stages of electroplating. The smut appears as a brown or black deposit on the surface after a short time in the electroplating bath. The smut is then subsequently overplated with the electroplated metal (usually copper). However the smut may and often does cause adhesion failures between the electroplated metal and the surface being plated particularly copper to copper adhesion, and in printed circuits interconnect defects. In addition this smut may interfere with electroplating in other ways, such as decreased efficiency, poor deposit integrity and other similar problems.
Thus it is an object of this invention to provide a process for direct electroplating on palladium, palladium-tin or similarly activated surfaces substantially without the formation of smut.
It is a further object of this invention to provide a process for direct electroplating on palladium, palladium-tin or similarly activated surfaces, which process provides improved adhesion of the plated metals to the substrate surfaces.
Finally it is an object of this invention to provide a process for direct electroplating on palladium, palladium-tin, or similarly activated surfaces which process provides improved plating efficiency and deposit integrity.